1. Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine, more particularly to an air/fuel ratio control system for an internal combustion engine in which control hunting is reduced by decreasing the effect of exhaust gas pulsation on the detected air/fuel ratio.
2. Description of the Prior Art
A number of techniques have been proposed for controlling the air/fuel ratio in an internal combustion engine based on the oxygen concentration of the exhaust gas from the engine measured using a sensor comprising an oxygen ion-conductive solid electrolyte material. As specific examples there can be mentioned in Japanese Laid-Open Patent Publication Nos. 61-272438 (U.S. Pat. No. 4,842,711) and 62-3143.
Sensors of this type generally have two bodies each composed of oxygen ion-conductive solid electrolyte material disposed opposite each other and each provided with a pair of electric terminals so as to constitute an oxygen-pumping element and a cell element for detecting oxygen concentration. More specifically, the space between the oxygen pumping element and the cell element is sealed off to form a gas diffusion chamber (diffusion restriction region). The wall of the chamber is provided with a slit for the introduction of exhaust gas, while ambient air is introduced on the opposite side of the cell element. The electromotive force developed between the terminals of the cell element is detected and compared with a reference voltage. A voltage proportional to the difference between the two voltages is applied across the oxygen-pumping element terminals so as to cause pumping current to flow from the external terminal toward the gas diffusion chamber terminal or vice versa and thus pump in or pump out oxygen ions. The pumping current is thus feedback controlled in the direction for reducing the difference between the electromotive force of the cell and the reference voltage. The pumping current value is converted to a voltage value proportional to the oxygen concentration. As a result it becomes possible to detect the air/fuel ratio over a wide range extending from a rich to a lean mixture.
However, the exhaust gas to which the oxygen concentration sensor of this type is exposed pulsates in a manner that changes as the operating condition of the engine changes and, as a result, the aforesaid pumping current, which is affected by the exhaust gas pulsation, also varies depending on the operating condition of the engine. When the raw value of the oxygen concentration detected by the sensor is used for controlling the air/fuel ratio, control hunting occurs. For reducing the effect of the exhaust gas pulsation on the detection value and thus suppressing control hunting it has been proposed to correct the detection value in accordance with the engine speed and engine load (Japanese Laid-Open Utility Model Publication No. 64-32442), to smooth the detection value in accordance with the engine speed and engine load (Japanese Laid-Open Patent Publication Nos. 62-96754 and 1-206251), and to vary a constant at each instant in accordance with the engine speed (Japanese Laid-Open Patent Publication Nos. 61-272439 (U.S. Pat. No. 4,767,520) and 61-294358).
Since these prior art technologies do not detect the change in exhaust gas pulsation with variation in various operating parameters and the operating condition of the engine, they are not able to prevent the control hunting to an adequate degree.
An object of this invention is therefore to eliminate the aforesaid problem by providing an air/fuel ratio control system for an internal combustion engine which reduces control hunting by decreasing the effect of exhaust gas pulsation on the detected air/fuel ratio.
Moreover, the prior art has not given adequate attention to the fact that the engine operating condition differs greatly between normal and transient (accelerating or decelerating) operation.
Another object of the invention is therefore to provide an air/fuel ratio control system for an internal combustion engine which is able to reduce control hunting by decreasing the effect of exhaust gas pulsation on the detected air/fuel ratio, irrespective of whether the engine is in a normal or a transient operating state.